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Silver-doped CdS quantum dots capped with mercaptoacetic acid were incorporated into a chitosan-coated filter paper to fabricate a colorimetric test stripe for mercury. Chitosan assisted in the stabilization of CdSAg QDs on cellulose and enhanced the analyte diffusion in the test stripe. Hg(II) ions were captured by mercaptoacetic acid on CdSAg, resulting in a visualized color change from yellow to deep brown. For enhanced sensitivity and quantitation with good reproducibility, the color intensity was recorded, transmitted to a smartphone camera and computed by the ImageJ software to provide a digital readout. The assay was rapid, simple, and selective for mercury as several potential interfering species provoked no signal response. The applicability of the sensing approach was demonstrated for the analysis of spiked Hg(II) ions in cosmetic cream with a detection limit of 124 μM. Graphical abstract Schematic of a colorimetric test stripe consisting of silver-doped CdS quantum dots coated withmercaptoacetic acid and embedded into a chitosan matrix on paper. It enables mercury(II) to be visuallyquantified even in complex matrices such as cosmetics.

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An L‐cysteine capped cadmium sulfide‐chitosan nanocomposite has been synthesized, characterized and used for surface modification of a glassy carbon electrode. With direct electron transfer, hemoglobin (Hb) adsorbed strongly on the nanocomposite and displayed excellent bioelectrocatalysis for H2O2. The biosensor was capable of reducing H2O2 at −0.35 V, with a detection limit of 3.13 nM, linearity in the range of 15 nM to 10 µM and a response time of less than 2 s. The MichaelisMenten constant (KM) was 0.57 nM, attesting high bioelectrocatalysis of immobilized Hb for H2O2. Reproducibility of the fabrication method was very satisfactory with a relative standard deviation of 5.3 %. The biosensor lost only 6.5 % of its original response after 7 days when stored in a pH 7.4 PBS at 4 °C.

A stable complex of silver nanoparticles (Ag NPs) capped by cysteamine (Cst) together with single‐walled carbon nanotube (CNTs) was used to modify a glassy carbon electrode (GCE) for simultaneous detection of hydroquinone (HQ) and catechol (CT). The resulting electrode (AgCst‐CNTs/GCE) showed excellent electrocatalysis and reversibility towards this electroactive pair. The peak separations of their oxidation‐reduction peaks decreased significantly, compared with those of the unmodified GCE. The signal responses of the AgCst‐CNTs/GCE were 5‐fold higher while its peak potential separation remained unchanged (ca. 130 mV), compared to the CNTs‐modified GCE. The oxidation peak currents obtained for HQ and CT exhibited linearly from submicromolar to hundred micromolar concentrations without any cross‐interference. The modified electrode possessed a very large active surface area with a detection limit (S/N=3) of 10 and 40 nM for HQ and CT, respectively. The sensor was demonstrated for the analysis of river water and topical cream as evinced by high accuracy and reproducibility.